I'm on a Medicare Part D Plan and Receive a Part D low-income Subsidy. I Received a Letter from the Social Security Administration that I no longer qualify for the subsidy. Can I switch to a lower cost Part D plan?

Yes, you qualify for a Special Election Period which begins the month you were notified of the loss and continues for two more months. This SEP allows you one opportunity to enroll into another PDP or an MA-PD Plan.